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Sensors and Actuators A 125 (2006) 367375 Injection molding and injection compression molding of three-beam grating of DVD pickup lens Cheng-Hsien Wu, Wei-Shiu Chen Department of Mechanical and Automation Engineering, Da-Yeh University, Chang-Hwa 51505, Taiwan Received 21 January 2005; received in revised form 8 July 2005; accepted 14 July 2005 Available online 5 October 2005 Abstract The objective of this paper is to investigate the application of injection molding and injection compression molding processes to produce diffraction gratings. A mold was designed to produce a diffraction rating connected with the fi xed bushing. The combined part was verifi ed to have a good diffraction performance. Integrated grating eliminates the assembly cost and error. Photolithography was applied to make the mold insert. The Taguchi method and parametric analysis were applied to study the effects of molding parameters on grating quality. The design, fabrication of structured mold surfaces and the results of the replication by injection molding (IM) and injection compression molding (ICM) are presented and compared. The diffraction angle of ICM grating is more accurate than that of IM grating. Grating made by ICM has a much smaller warpage than that made by IM. The diffraction pattern shows that ICM is a better process than IM to replicate a diffraction grating. 2005 Elsevier B.V. All rights reserved. Keywords: Injection molding; Injection compression molding; Grating; Replication 1. Introduction All parts in an optical pickup, including the recording sub- strateandanumberofcomponents,arerequiredtohavesuperior optical performance. This requires a very accurate shape repli- cation and low optical anisotropy as induced by the molding related stresses. Whenever a travelling wave encounters an obstruction with dimensions similar to its wavelength, some of the energy in the wave is scattered 1,2. If the obstruction is periodic, or indeed ifthereisaperiodicvariationofanyparameterwhichaffectsthe propagationofthewave,energyisscatteredintovariousdiscrete directions or diffracted orders, and a structure which acts in this way may be referred to as a diffraction grating. Intheopticalpickupoftheplayer,diffractiveopticalelements (DOE)areusedtosplitthemainlaserbeamintothreebeamsfor track following but also to defl ect the returning beam onto the detector area. The classical method of manufacturing gratings is to scribe, burnish or emboss a series of grooves upon a good optical surface 3. Corresponding author. Tel.: +886 4 8511227; fax: +886 4 8511224. E-mail address: .tw (C.-H. Wu). The master is usually a metal copy of a DOE original fabri- cated by one or a combination of high-resolution lithographic steps 4,5. The material in which the original is fabricated can be resist, quartz, silicon or virtually any suitable high-resolution fi lm or substrate. The master is typically fabricated from the original by electroplating in nickel although, in certain cases, it can be another material such as quartz, plastic or another metal, or even the original itself. The master grating is fi rst coated with a thin layer of some non-adherent material (e.g. gold). This is followed by a substantial layer of aluminum. The master is then cementedwiththeaidofathinfi lmofalowviscosityresintothe carefullycleanedreplicablank,allowingtheresintopolymerize at a constant temperature, generally a slow process. When the resin has cured, the replica and the master are separated. How- ever,theprocesstakestoomuchtimeandisnotsuitableformass production. To be a major technology for the low-cost, mass production of DOEs, alternative replication methods are applied as follows. There are three alternative replication methods to replicate: the nickel master casting, embossing and injection molding (IM). In casting, a thin fi lm of epoxy is applied on a solid sub- strate blank. Both room temperature and thermally cured epox- ies have been used. The main disadvantage is the long curing time. A related technique of particular interest for DOEs is the 0924-4247/$ see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2005.07.025 368C.-H. Wu, W.-S. Chen / Sensors and Actuators A 125 (2006) 367375 replication into a thin fi lm of UV-curable material coated onto a glass or polymer substrate. An even faster method of making grating replicas involves embossing a plastic fi lm by passing it over a heated cylinder under some pressure from a smooth back-up roll. The cylinder will typically have a nickel electroplated wrapping around it. Since this is a continuous process the unit cost will be mini- mal,butqualityislimitedtostudentexperiments,ormorelikely decorative devices such as holograms. Injection molding is a classic low cost process, and in prin- ciple could produce gratings by inserting a nickel electroplated replica derived from a precision master into appropriate molds. The accuracy of injection molded gratings is always limited by the molding conditions of the process. Efforts need to be made to identify the signifi cant factors that affect micro-fi lling behav- iors. In recent years, plastics have begun to show great com- mercial potential especially in manufacturing micro-structured parts. Injection molding represents the most important process for manufacturing plastic parts. While many prototype plas- tic micro-devices are fabricated using precision engineering methods such as laser machining, micro-injection molding is currently being investigated all over the world 6,7. An impor- tant advantage of injection molding is that with it we can make complex geometries in one production step in an auto- matedprocess.Manymicro-devicessuchaswatchesandcamera components, automotive crash, acceleration, distance sensors, read/write heads of hard discs, CD drives, medical sensors, pumps, surgical instruments and telecommunications compo- nents, have been successfully injection molded. Theinjectionmoldingprocessinvolvestheinjectionofamelt polymer into a mold where the melt cools and solidifi es to form a plastic part. It is generally a three-phase process including fi lling, packing and cooling phases. After the cavity becomes stable, the product is ejected from the mold. Despite many advantages, the injection molding process experiences some inherent problems in molding micro-features 8. The main diffi culty is that the molten polymer in a tiny cavity instantaneously freezes upon touching the relatively cold cavity wall. The problem gets worse when micro-features with high aspect ratios are to be molded. The best replication results were achieved when melt and mold temperatures were higher than normal values 9. Injection compression molding (ICM) has the advantages such as decreasing molding pressure, reducing residual stress, minimizing molecular orientation, packing evenly, reducing unevenshrinkage,overcomingsinkmarkandwarpage,reducing density variation and increasing dimensional accuracy. Because of these advantages, injection compression molding is often employed to produce parts of high accurate dimension and free of residual stress especially for the optical parts. Instead of using a micro-injection molding machine, Wimberger-Friedl 7 fabricated sub-?m grating optical ele- ments by conventional injection molding. Inserts were made by two different processes: reactive-ion etching (RIE) in SiO2and electron-beamlithographyfollowedbynickelelectroplating.No fi lling problem was encountered with a mold temperature of 140C. However, SEM micrographs show that there is a shape deviation, i.e. the structures are thicker at the top than at the bottom. Grating elements were designed to have a pitch below 0.5?m and depths in excess of 1.5?m. By splitting the func- tion over the two surfaces of the retardation elements, only a half fi lling depth was necessary. The effect of shape deviation can be reduced. Grating structures with a record depth of 2?m and a pitch of 0.5?m were replicated by injection molding with polycarbonate. Features with high aspect ratio proved diffi cult to form rou- tinely however it is envisaged that further optimization of injec- tion molding will resolve these diffi culties 10. Parashar et al. 11 employed a two-step pattern transfer to replicate the structure in glass: fi rst a polydimethylsiloxane (PDMS) replica is obtained from a master structure and secondly, a layer of solgel material is applied on the PDMS soft-replica to get micro-/nano-structure in glass after drying and annealing. They haveestablishedtheeffectivenessofin-housedevelopedsolgel materials derived from metal alkoxides in nano-replication of glass micro-/nano-structure. Obi et al. 12 presented a replication method to fabricate micro-structures. UV-curable solgel is used as base material. Thebasicfabricationprocessinvolvesdepositionandpatterning of a sacrifi cial spacer layer and a combined molding and pho- tolithography step. This fabrication process can be applied for opticalMEMSdevicesthatincorporatelenses,diffractiveoptics or waveguides. The objective of this paper is to investigate the application of IM and ICM processes to produce diffraction gratings. The Taguchi method and parametric analysis were applied to study the effects of molding parameters on the grating quality. The design, fabrication of structured mold surfaces and the results of the replication by IM and ICM are presented. 2. Transmission diffraction grating 2.1. Application exampleoptical pickup When the laser beam goes through the diffraction grating, it is split up into a central bright beam plus a number of side beams. The central beam and one beam on each side are used by the CD for the tracking system. Consider a segment of the CD player containing several tracks. If the optical head is on track, then the primary beam will be centered on a track (with pits and bumps) and the two secondary beams will be centered on land. The three spots are deliberately offset approximately 20?m with respect to each other. Two additional detectors are placed alongside the main quadrant detector in order to pickup these subsidiary beams. If the three beams are on track, then the two subsidiary photodetectors have equal amounts of light and will be quite bright because they are only tracking on land. The central beam will be reduced in brightness because it is tracking on both land and pits. However, if the optical head is off track, then the center spot gets more light (because there are fewer pits off track) and the side detectors will be misbalanced. The most common optical train in modern CD players is the three-beampickup,depictedinFig.1.Thelightisemittedbythe C.-H. Wu, W.-S. Chen / Sensors and Actuators A 125 (2006) 367375369 Fig. 1. Schematic illustration of an optical pickup. laser diode and enters a diffraction grating. The grating converts the light into a central peak plus side peaks. The main central peak and two side peaks are important in the tracking mech- anism. The three beams go through a polarizing beam splitter. Thisonlytransmitspolarizationsparalleltothepage.Theemerg- ing light (now polarized parallel to the page) is then collimated. The collimated light goes through a /4 plate. This converts it into circularly polarized light. The circularly polarized light is then focused down onto the disk. If the light strikes land it is refl ected back into the objective lens. (If the light strikes the pit, now a bump, it is not refl ected.) The light then passes through the /4 plate again. Since it is going in the reverse direction, it will be polarized perpendicular to the original beam (in other words, the light polarization is now vertical with respect to the paper). When the vertically polarized light hits the polarizing beam splitter this time, it will be refl ected (not transmitted as before). Thus, it will refl ect through the focusing lens and then the cylindrical lens and be imaged on the photodetector array. The cylindrical lens is important in the auto-focusing mecha- nism. 2.2. The general grating equation Atransmissiondiffractiongratingisaslidewithlargenumber of parallel, closely spaced slits (transparent spaces) drawn on it. Early ones were carbon covered glass slides etched by a needle pointnow they tend to be printed onto a slide. It is excellent at separating the colors in incident light because different wave- lengths are diffracted at different angles (Fig. 2), according to the grating relationship: d sin = n(1) where d is the distance between the slits, the angle of diffrac- tion,thewavelengthofthelightandnistheorderofdiffraction. 2.3. Diffraction effi ciency Diffraction effi ciencies of standard phase gratings, contin- uously blazed or multilevel approximations, are a fundamental issueindiffractivemicro-optics.Innearlyallapplicationswhere gratingsareused,ahighdiffractioneffi ciencyisamajorrequire- ment. Achievable effi ciency in practice depends on numerous Fig. 2. A transmission grating. factors: the type and performance of the fabrication technol- ogy used, the grating period to wavelength ratio, the materials, etc. Additionally, the actual value depends on the defi nition of diffraction effi ciency, since a unique defi nition does not exist. Therefore, it is often quite diffi cult to compare results which appear in the literature. In order to characterize the performance of diffraction grat- ings, two numbers are often used: (1) The overall effi ciency o,1 Thisoveralleffi ciencyisdefi nedaso,1 =intensityoffi rst order/intensity of incident beam. (2) The diffraction effi ciency d,1 The diffraction effi ciency is defi ned as d,1=intensity of fi rst order/intensity of transmitted beam through unstruc- tured substrate. With the second defi nition, losses due to scattering from surface roughness at the grating interface are contained in the measurement of effi ciency. In this study, the diffraction effi - ciency d,1is used. 3. Experimental procedures 3.1. Material The material used in this study is a high heat injection grade of polymethylmethacrylate (PMMA, CM-205, from Chi Mei Corp.,Taiwan).Themeltfl owindexis1.8g/10minandthebulk density is 1.19g/cm3. The recommended barrel temperature is between 210 and 250C and the recommended mold tempera- ture between 50 and 70C. The material was pre-dried at 90C for 4h using a dehumidifying drier before molding. 3.2. Part geometry and mold design Diffractiongratingsweremachinedonaglasssubstrate.After machining,thegratinghastobeassembledintoafi xingbushing 370C.-H. Wu, W.-S. Chen / Sensors and Actuators A 125 (2006) 367375 Fig. 3. The grating and the fi xing bushing. (as shown in Fig. 3). Assembly requires much production time and labor cost. It also creates positioning error and angle error, andthenreducestheaccuracyofanopticalpickup.Inthisstudy, the product was designed to combine grating and bushing. The whole part has a diameter of 7mm. The grating portion, with a diameterof4mm,islocatedatthecenter.Thenickelmoldinsert was designed with a periodicity of 20?m. The notch depth was 1.5?m. Theelectroplatednickelmoldinsertwasinstalledinacenter- gated mold base as shown in Fig. 4. The mold has two cavities symmetrically located on the opposite sides of the sprue. The mold plates are made of S45C tool steel. The cavity is fed by a sprue, two runners and two fan gates as shown in Fig. 5. The sprue is 48mm long and 5mm in diameter. The dimensions of runners are 1.55mm3.50mm5.68mm. 3.3. Mold insert fabrication Our photolithography process involves photomask fabrica- tion, wafer cleaning, spin coating, soft baking, exposure, post- exposure baking, developing and hard baking. A high-resolution transparency containing the features design, created in a CAD program, was used as the mask in photolithography. In this study, the mask pattern was trans- ferred onto a transparency using a high-resolution laser printer (16,000dpi). Fig. 6 shows the schematic of the fabrication steps required to produce a nickel electroplated mold insert. A silicon wafer was used as the substrate. Wafer cleaning is a necessary step to Fig. 4. A center-gated mold base with an electroplated nickel mold insert. Fig. 5. Schematic illustration of cavity arrangement. remove contaminants from the wafer surface in order to obtain high performance and high reliability of products, and to pre- ventcontaminationofprocessequipment.Thewafersurfacewas cleaned with a 4:1 H2SO4/H2O2for 10min at 120C to remove organiccontaminants.Thewafersurfacewasrinsedusingdeion- ized(DI)wateruntilthewaterresistancewashigherthan8?.A 50:1 H2O/HF step for 10min at room temperature was applied to remove chemical oxides. The wafer surface was rinsed using deionized water again. After wafer cleaning, the substrate was spun, blown dry using heated nitrogen and then placed on a hot plate (120C for 3min) to drive off any water vapor on the surface. This step is called dehydration baking. To improve the adhesion of resists to the silicon wafer, hex- amethyldisilane(HMDS)isoftenapplied.HMDSwasappliedto thewaferbyspinningatroomtemperature.HMDSwasdriedby placing the wafer on a hot plate for 2min at 90C. The next step isspinningtheresistontothewaferanditshouldbedoneimme- diately after the HMDS application. A positive resist, AZ9260, was used in this study. The resist is dispensed onto the wafer while the wafer is spinning to produce a uniform layer on the wafer.Thespinspeedofthespincoaterwasincreasedto500rpm Fig. 6. Fabrication schematic to produce nickel electroplated mold inserts. C.-H. Wu, W.-S. Chen / Sensors and Actuators A 125 (2006) 367375371 with an acceleration of 500rpm/s for 10s. For another 30s, the spin speed and acceleration of the spin coater were 300rpm and 300rpm/s, respectively. The spin speed at this step determines the fi nal thickness of the resist (about 50?m in this study). The next step in the lithography is the pre-bake. Pre-bake conditions depend on the thickness of